1. Field of the Invention
The present invention relates to a packaging material for a lithium ion battery, a lithium ion battery, and a method for manufacturing a lithium ion battery.
2. Description of the Related Art
As a secondary battery for consumer use which is used in a PC, a portable terminal device such as a cellular phone, a video camera, and the like, a lithium ion battery which has high energy and is capable of being made slimmer and more compact has been actively developed. As a packaging material for a lithium ion battery (hereinafter, may be simply referred to as “packaging material”), a deep-drawn molded product, which is obtained by deep drawing a multi-layer laminated film (for example, a configuration such as a heat-resistant base material layer/an aluminum foil layer/a sealant (heat-fusible film)) by cold molding (deep drawing), is used in place of a metal can of the related art with an advantage that weight is light and a battery shape may be freely selected. In addition, with the advantages that the packaging material using the laminated film has a degree of freedom in battery shape, light weight, and high heat dissipation, and is cheaper, the packaging material has been attempted to be applied to batteries for recently developed hybrid cars and electric vehicles in which environmental load is less.
In the lithium ion battery using the laminated film type packaging material, an electrolyte layer which is formed from an electrolytic solution obtained by dissolving a lithium salt in an aprotic solvent (propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, or the like), or a polymer gel to which the electrolytic solution is impregnated is accommodated in the deep drawn molded product together with a positive electrode material, a negative electrode material, and a separator as main-body portions of the battery. Then, the deep drawn product is heat-sealed by heat sealing.
The electrolytic solution has high permeability with respect to the sealant layer. Therefore, the permeated electrolytic solution decreases laminate strength between the aluminum foil layer and the sealant layer, and ultimately, the electrolytic solution may be leaked in some cases. In addition, lithium salts such as LiPF6 and LiBF4 that are electrolytes generate hydrofluoric acid by hydrolysis reaction, and thus corrosion of a metal surface and a decrease in laminate strength between respective layers of the laminated film are caused. Therefore, it is necessary for the packaging material to have a performance capable of preventing corrosion against the electrolytic solution or the hydrofluoric acid.
As a method for providing performance capable of preventing corrosion against the electrolytic solution or the hydrofluoric acid, a method in which a chromate treatment using hexavalent chromium is carried out with respect to an aluminum foil surface is known. However, as can be seen in Rohs restriction or REACH restriction in Europe, hexavalent chromium is treated as an environmentally harmful material, and thus a chromate treatment using trivalent chromium is carried out. However, since hexavalent chromium is used as a starting material to obtain trivalent chromium, total abolition of chromium may be enforced in the future. Particularly, in consideration of the application to electric vehicles considering an effect on the environment, it is important to provide performance capable of preventing corrosion against the electrolytic solution or the hydrofluoric acid by a treatment not using a chromium component at all.
On the other hand, excellent moldability is required for the packaging material. That is, an energy density is determined by the number of cells and amount of electrolytic solution that can be accommodated in the lithium ion battery. Accordingly, during molding of the packaging material into a battery shape, it is necessary to make the molding depth deep so as to increase an accommodated number of cells and amount of electrolytic solution.
Generally, the molding of the packaging material is carried out by cold molding (deep drawing molding) using a mold. However, at this time, when the molding depth is too deep, cracking or a pinhole occurs at a stretched portion of the molding, and thus reliability as a battery disappears. Therefore, it is important to make the molding depth deep without deteriorating reliability.
Particularly, in large-scale application for electric vehicles and the like, it is desired to further increase the energy density from an aspect of a battery performance of taking out a large current. On the other hand, excellent reliability and long-term storage stability are also required at the same time.
In addition, it is necessary for the above-described heat-resistant base material layer to have excellent chemical resistance and scratch resistance. As the base material layer, a polyamide film is frequently used in consideration of moldability. However, the polyamide film is dissolved in an electrolytic solution containing a lithium salt. Therefore, when the electrolytic solution adheres to the base material layer of the packaging material by accident during manufacturing of the battery, the polyamide film is corroded, and thus this corrosion has an effect on a battery manufacturing yield rate. In addition, in a use for electric vehicles, an assembled battery in which a plurality of battery cells are integrated is used to increase output. In the assembled battery, there is a concern that adjacent battery cells may scratch each other due to vibration during vehicle driving, and thus the base material layer may be damaged. Furthermore, when the electrolytic solution is leaked due to an effect of the damage, the electrolytic solution adheres to another battery cell, and thus the assembled battery may be extensively damaged.
Therefore, as a packaging material in which electrical solution resistance and scratch resistance are provided to the surface of the base material layer, a packaging material having the following structure is known.
(1) A packaging material in which a first base material film layer, a second base material film layer, a metal foil layer, and a heat-adhesive resin layer are laminated in this order from the outside is known (Japanese Patent No. 4559547 ((hereinafeter, Patent Document 1)). In the packaging material, the first base material film layer is made of a biaxially stretched polyethylene terephthalate film (hereinafter, referred to as a “biaxially stretched PET film”), and the second base material film layer is made of a biaxially stretched nylon film (hereinafter, referred to as a “biaxially stretched Ny film”) (Patent Document 1). The packaging material has a structure in which the biaxially stretched PET film having low hygroscopicity, rigidity, scratch resistance, and heat resistance, and the biaxially stretched Ny film having flexibility, pricking strength, bending strength, and low-temperature resistance are bonded to each other by a known dry laminate method using a two-liquid curing type polyurethane-based adhesive and the like. The packaging material having this structure also has the characteristics of the above-described film.
(2) A packaging material in which a coating layer formed from a specific resin such as polyvinylidene chloride and polyvinylidene chloride-vinyl chloride copolymer is formed on a surface side of a stretched film on which a base material layer is formed is known (Japanese Patent No. 3567229 (hereinafeter, Patent Document 2)). The stretched film is protected by the coating layer.
However, in the packaging materials (1) and (2), a portion stretched by molding has a tendency to return to its original shape, and thus the molding depth substantially decreases, or a shape varies in some cases.